The present invention concern rotary printing presses for single sheets, for direct or indirect printing of sheets on one side only or on both sides.
Rotary printing presses are known in which sheets of paper, card or other printable material, withdrawn singly from a pile by means of a feeder, are caused to travel, partly overlapping, along a feeder table by means of belts or like conveyor means, and then stopped so as to be set in register frontally and laterally at the end of their travel upon the feeder table, after a control device has signalled their arrival in the correct position, the setting of the sheets in register being so effected that the sheets are all in the same position in relation to the printing members of the press and, in particular, in relation to the printing cylinder or forme, which is the element which transfers ink on to the sheet according to the graphical signs to be printed.
The printing cylinder receives ink from inking rolls. The sheet, after being checked and set in register upon the feeder table, is gripped by suitable feeder members and moved from the rest condition to the peripheral velocity of the printing cylinder, on which it is deposited to receive a printing of ink therefrom. From the printing cylinder the sheet is delivered to extractor members which convey it to a stack upon a delivery table. It is possible to insert upstream of the delivery table additional printing units arranged to print upon the sheet different graphical patterns by means of respective printing cylinders, the sheet being conveyed from one printing cylinder to the next by means of a series of conveyor rolls.
In present-day presses of this type, the need to make them even faster and more convenient presents problems which, if not new, are nevertheless acute.
More particularly, reduction of the diameter of the printing cylinders for a given maximum format of the sheet to be printed is a goal towards which manufacturers are striving, in view of the considerable advantages to be derived therefrom.
Reducing the diameter of the printing cylinder, in fact, makes it possible, in the first place, to reduce the peripheral speed at which ink is transferred from the inking rolls to the printing cylinder and from the latter to the sheet, and conversely to increase the angular velocity of the cylinder for a given optimal peripheral velocity, and hence increase the output of printed sheets in a given unit of time. Also it will be noted that a reduction of the peripheral velocity of the printing cylinder makes the control of the sheet easier.
In the second place, reduction of the diameter of the printing cylinder allows, especially in large size printing presses, reduction of the weight of the press, and hence of its cost, as well as its overall dimensions, so as to render it easier of access and more convenient.
Moreover, since the movement of inertia of a rotating cylindrical mass is inversely proportional to the square of its radius, reduction of the diameter of the printing cylinder and of the other rotary members the dimensions of which are determined by the diameter of the printing cylinder, such as the members conveying the sheet from the feeder table of the machine to the delivery table, entails a reduction of the related inertial forces of inertia and of the consequent mechanical stresses. This reduction of the inertial forces is an advantage which is the more effective as the maximum angular velocity required of the printing cylinder increases, since it makes it possible to prolong the life of those members which are under the most stress, such as shafts, bushes and, especially, gears. Particularly in offset printing presses the maintainance of the drive gears at peak efficiency is very important in order to achieve perfectly successful printing.
The reduction of the printing cylinder diameter, however, leads, at a given maximum printing format, to an increase in the angular width of the printing arc of the cylinder and a proportional reduction in the non-printing arc which, in hitherto known presses, is linked the stopping time of the sheet on the feeder table.
This stopping time is necessary, in known systems, in order to carry out the following operations on the sheet: detection of regular arrival of the sheets at the registers; front registration of the sheets; lateral registration; gripping of the sheet by the feeder members when the sheet is at rest, and disengagement of the edge of the stationary sheet from the register members.
Since, with systems at present in use, in order to carry out all, or almost all, these operations, with the sheets fed in overlapping relation, it is absolutely necessary that the front edge of the sheet arriving at the registers should be uncovered by the tail portion of the preceding sheet which is going on for printing. The speed of removal of the said tail portion of the preceding sheet therefore takes on great importance.
This speed of removal depends, for a given maximum format of sheet to be printed and for a given diameter of the printing cylinder, upon the distance which the sheet has to travel in passing from the feeder table to the said cylinder, and upon the movement imparted to the sheets by the sheet feeding members.
The types of sheet feeding members known up to the present are confined to withdrawing the sheet, while stationary and already set upon the feeder table and to increasing its speed up to the constant peripheral velocity of the printing cylinder (or of an equivalent intermediate transfer cylinder) without ever exceeding this velocity.
More precisely, up until now, for these purposes the following types of sheet feeding members have been employed:
a. Grippers with rotary drive housing, carried by the printing cylinder and arranged in each rotation to grip the edge of the stationary sheet, carrying it immediately to the peripheral velocity of the said cylinder. This constitutes a very simple system, with only small masses used, but is antiquated and unacceptable since it gives no guarantee as to the precision of print registration, especially if the velocity of the machine is even slightly varied.
b. Lever grippers, consisting of a series of grippers which, carried upon lever arms, grip the stationary sheet when set in register upon the feeder table and carry it quickly to the peripheral velocity of the printing cylinder (or of an intermediate transfer cylinder the peripheral velocity of which is equal to that of the printing cylinder), transfer the sheet to the latter at a tangent and then decelerate to stop at a dead centre position, from which position they return to take up the next sheet.
The circular path of the grippers is tangential to the feeder table and to the printing cylinder (or to the intermediate transfer cylinder) and is disposed mostly above the printing cylinder and the feeder table, but can also be below them, giving rise in this latter case to the so-called "English" feed system.
Lever grippers have the advantage that they are open in the return path and in a position to take up the next sheet ready on the feeder table, but they have the disadvantage of having to pass back, on returning, in correspondence with the printing cylinder only after having passed the end of the sheet of the largest format carried by the press on to printing by the printing cylinder. If the largest format is of considerable dimensions in relation to the diameter of the cylinder, then little time is left to complete the return of the lever grippers on to the feeder table, and their deceleration on arrival at the feeder table is too sudden.
This disadvantage is avoided by movement of the centre of oscillation (for example by means of a pair of eccentric bushes pivoted to the shoulders of the cylinder and driven by a suitable mechanism) so as to detach the lever grippers from the cylinder at the critical moment of their passage, so that they do not intrude into the printing area of the cylinder.
c. Intermittently rotating drum provided with a series of grippers, carried either by a drum or by a series of arms which, up to the transfer of the sheet to the printing cylinder, behave like the lever grippers, but after this continue their circular movement, in the same rotating direction, decelerating at the end of the revolution of the drum to come to a stop at a tangent to the feeder table, for withdrawing the next sheet.
This device has the advantage of not having a reversing movement, and therefore it does not pass back in correspondence with the printing cylinder, as occurs in the case of lever grippers on their return. It has nevertheless the disadvantage of coming down onto the sheet on the feeder table, and if the sheet is already stationary on the table in the position of register, it is necessary to provide some device whereby the gripper shall not disturb the registration of the sheet (for example, the feeder table can be lowered and then raised).
d. Continuously rotating drum. This device has been embodied in two systems, as follows:
In the first system the drum receives, directly, a sheet which has stopped for setting upon the feeder table, whilst still rotating continuously. It carries a turnover group of take-off grippers driven so that this group retracts within the periphery of the drum when the grippers pass over the stationary sheet, and then emerge again at a drive rate such that the peripheral velocity of the drum, combined with the equal and opposite turn-over speed of the gripper group, shall give, at one instant, zero resultant velocity when the grippers take hold of the sheet edge.
In the second "feed rolls" pointing system the sheet is controlled, and is stopped upon the feeder table, against a series of fixed front registers. There then follows the lateral registration of the sheet on the feeder table. Subsequently a group of upper and lower feed rolls speed up the sheet and thrust its front edge against a second series of front registers carried upon a continuously rotating drum. The grippers of the drum take up the sheet thus set and exchange it with the printing cylinder at a tangent position and at equal peripheral velocity.
Acceleration of the sheet can be effected partly by the "feed rolls" and partly by the drum if the latter revolves at a variable velocity, or else entirely by the "feed rolls" if the drum revolves at a peripheral velocity which is constantly equal to that of the printing cylinder; in the latter case the drum becomes an intermediate transfer cylinder, and, if desired, feed-in by the "feed rolls" could be effected directly against the printing cylinder.
Sheet feed by means of "feed rolls" has the advantage of eliminating the considerable masses of the lever grippers, as well as the problems related to their acceleration, but it has the disadvantage of not being suitable for all types of paper, and moreover it causes, in the front edge of the sheet, deformations which in the printing process are amplified towards the tail end, and can badly affect good setting.
All the traditional sheet feed means considered up to now have, in order of diminution of the diameter of the cylinders for a given format of the sheet to be printed, considerable limitations and weak points. On average, the time the sheet is required in practice to remain stationary on the feeder table corresponds to about 90.degree. - 100.degree. of rotation of the printing cylinder.
By the use of various types of devices known as "preregisters", especially in large format printing presses, the stopping time has been reduced to about 60.degree. of rotation of the printing cylinder, and consequently it has been possible to reduce the diameter of the said cylinder for a given maximum format of paper.
Such known devices consist substantially of a series of frontally aligned locators which move periodically, in correspondence with the arrival of every sheet, in the direction of advantage of the sheet-guide belts. The speed at which the frontal locators move is slightly less, at the beginning, than that of the belts, and decreases progressively so as to facilitate the setting in register, against the said locators, of the front edge of the sheet in its descent. When the sheet comes, with reduced impact velocity, into contact with the main (fixed) registers, its front setting is almost complete, and the stopping time of the sheet is therefore practically reduced to the time of lateral registration and to the time of checking the arrival of the sheet. The disadvantage of known pre-register devices is their complexity. Moreover, in the way such devices have been used up to now, the reduction in the stopping time of the sheet on the feeder table renders the pre-register devices unsuitable for use upon machines with a very high production speed. In fact, in this case, owing to the increased rotational speed, the already low stopping time is further reduced by the lesser time taken to effect each turn of the printing cylinder. Beyond a certain limit of velocity, the stopping time becomes insufficient in the proper sense for carrying out sure end perfect setting of the sheet, inasmuch as the sheet, especially one of large format, no longer has the time to settle itself against the pre-registers.
Also, checking the regular arrival of the sheets at the registers can be impaired by excessive brevity of the stopping time, since, in the event of irregular arrival of a sheet, it is necessary to provide a signal for operating electrical, mechanical, hydraulic or pneumatic servocontrols in sufficient time to prevent printing of the sheet, and to effect, before the printing cycle starts, different sheet delivery operations, including removal of pressure between the cylinders, interruption of the sheet feed, removal of the inking rollers from the printing cylinder, and stopping of the setters on the table.
In order to overcome the disadvantages and shortcomings which conventional sheet feeding systems possess, the most advantageous solution is that of advancing disengagement of the line of registers from the end of the sheet of greatest format which is going to be printed.
Attempts have been made in this direction, with the intention of speeding up the tail of the sheet which is going to be printed (using suitable acceleration devices, such as, for example, a series of sucker rollers placed below the feed table) after the said sheet has been grasped by the grippers of the printing cylinder, making them form a pocket underneath the said cylinder. The tail of the sheet, however, after having been left by the acceleration devices, hangs free, and is thus able to come into contact with the neighbouring surfaces.
Such a solution is not acceptable owing to the serious danger of spoiling the sheet and damaging printing which may previously have been done, especially in fast and large size machines.
A main object of this invention is the avoidance of the difficulties, shortcomings and the disadvantages which are mentioned above.